The present invention relates generally to the field of earth boring bits and, more particularly, to roller cone rock bits such as those typically used in oil and gas exploration.
Roller cone rock bits generally comprise a main bit body which can be attached to a rotary drill string. The bit body usually includes two or three legs which extend downward. Each leg has a journal extending at a downward and inward angle. A roller cone, with cutters, "teeth", or ridges positioned on its outer surface, is rotatably mounted on each journal. During drilling, the rotation of the drill string produces rotation of each roller cone about its journal thereby causing the cutter elements to engage and disintegrate the rock.
Because of their aggressive cutting action and resultantly faster penetration rates, roller cone rock bits have been widely used for oil, gas, and geothermal drilling operations. However, certain problems exist which limit the useful life and effectiveness of roller cone rock bits. The useful life of a rock bit is an especially critical consideration when viewed in light of the great expense in time and money required to remove and replace the entire drilling string because of bit failure.
The bearings employed between the cones and the journals are the source of significant problems. These bearings operate in an extremely hostile environment due to high and uneven loads, elevated temperatures and pressures, and the presence of abrasive grit both in the hole cuttings and the drilling fluid. This is particularly true when drilling deep holes. In addition, some rock bits such as those used in geothermal exploration are subject to corrosive chemical environments. Another factor which can lead to early bearing failure is the inability of the bearings to withstand changes in the moment of forces directed against the roller cone. As the inserts on the gage row, i.e. the row which engages the sides of the hole, gradually wear down, the sides of the hole become less defined. As a result, the forces from the side of the hole increase. These increased side forces tend to push the cone off its original axis of rotation, thereby "pinching" the bearings in their races and contributing to early bearing failure.
It has been observed that these extreme conditions often cause failure of the roller cone bearings before any other part of the bit, even before the cone's cutters. In addition, as the bearings wear, they can allow for more "wobble" of the cones. As a result, a roller cone bit with worn bearings does not track as well in the hole and has a reduced penetration rate. Also, these limits on the bearing's capacity in turn limit both the load which can be applied to the bit as well as the angular velocity at which the bit can be rotated, thereby establishing constraints on achievable penetration rates and feasible cutter designs.
In some of the earlier roller cone bit designs, the bearing structure was relatively simple. For example, U.S. Pat. Nos. 1,649,858 and 1,909,078 both show a roller cone rock bit with a frusto-conically shaped friction type bushing located between the journal and the cone. Such bearings had a relatively short life expectancy, but so did the other components of the early bits. However, as harder and longer lasting materials such as cemented tungsten carbide began to be used for some of the other components of the roller cone bits, and as these new bits were used to drill increasingly deeper holes through harder materials, several changes were made to improve the bearing's capacity to handle higher loads for longer periods of time.
At present, typical roller cone rock bits use bearings between the journal and roller cone which consist of combinations of anti-friction and friction bearings. The anti-friction bearings (such as ball or roller bearings) are used to facilitate rotation, absorb radially directed forces, and often to retain the roller cone on the journal. The friction bearings are used as "thrust" bearings where the mating surfaces are disposed perpendicular to the rotational axis of the cone and absorb axially directed forces. The friction bearings are also used as radial bearings where the mating surfaces are disposed parallel to the rotational axis and absorb radially directed forces.
Lubricants and coolants are frequently used to increase the life of bearings. One design uses a grease to lubricate the bearings. In bits using such a lubricant, it is necessary to seal the bearings in order to preserve the grease and keep out the drilling mud and rock cuttings. Most of these bits also incorporate a lubricant reservoir and pressure compensator to allow for lubricant loss as well as the high temperatures the bit may encounter in drilling deep holes. For example, see U.S. Pat. Nos. 3,397,928; 3,476,195; and 4,061,376. Naturally, each of these features adds complexity to the bit design. For example, fabrication usually requiring the cones to first be mounted or "pinned" on the journals on separate legs after which the legs are welded onto a main bit body. In addition, because of the harmful effects of high temperature on the seals and lubricants, the maximum rotational speed at which these sealed bearing bits are operated is often limited.
Attempts to increase bearing life have also been made involving the use of harder, more wear-resistant materials on the bearing surfaces. For example, most bits now employ carburizing, hard facing, or special metal inlays for the bearing surfaces, all of which increase the complexity and cost of fabrication. See, for example, U.S. Pat. No. 4,054,426. In addition, U.S. Pat. No. 4,190,301 teaches the use of a pair of opposing polycrystalline diamond compacts for the "nose" thrust bearing.
Also, U.S. Pat. No. 4,260,203 teaches the use of radial and thrust bearing surfaces consisting of polycrystalline diamond. Although the use of polycrystalline diamond bearing surfaces may be an improvement over the use of other materials, the design of the '203 bit does not account for certain properties inherent in polycrystalline diamond. In particular, it has been observed that polycrystalline diamond, although very strong in compression, is not very strong in tension. Because the '203 design has both radial and axial bearings set perpendicular to each other it will subject the polycrystalline diamond to substantial tensile forces. That is, as the thrust bearings are put in compression, the radial bearings are put in tension. Also, the patent does not disclose a method of retaining the cone on the journal other than the conventional method with ball bearings which appears to leave a weak point in the bearing system.
U.S. Pat. No. 4,145,094 shows a somewhat simplified bearing system for roller cone rock bits which does not use ball bearings to retain the cone on the journal. In that design, the cone is retained on the journal by electron beam welding an annular thrust member into the journal after it has been inserted into the cone, thereby "plugging" the cone onto the journal.